1. Field of the Invention
This invention relates to ink for stencil printing, and more particularly to a stencil printing ink containing therein smectite.
2. Description of the Related Art
The stencil printing is a method of printing in which printing is made on paper or the like by transferring ink from one side of a stencil to the other side through imagewise perforations in the stencil.
It is generally preferred that the stencil printing ink can readily permeate the stencil and has moderate flowability as well as that it is excellent in leveling after printing and can dry soon after printing. Especially in the case of the ink for the stencil printing where the drum is of an open type, it is necessary that the ink has moderate hardness to prevent sag of the ink and at the same time the ink has moderate softness in order to promote penetration into the printing material. Accordingly, it is very important to adjust the flowability of the ink.
In order to adjust the flowability of the ink, a flow control agent has been often added to the ink. For example, for the stencil printing ink, smectite group clay minerals such as bentonite, hectorite, heide llite, saponite and the like containing montmorillonite as a main component have been often employed. For example, in Japanese Patent Publication No. 54(1979)-23601, there is disclosed stencil printing ink in which a thickening agent such as organic bentonite is added to stencil printing emulsion ink composition to increase effective hardness of the ink and improve finish of printing. In Japanese Unexamined Patent Publication No. 6(1994)-33007, there is disclosed emulsion ink which contains montmorillonite clathrated surfactant.
The flow control effect of the smectite is considered to be governed by the particle size of the smectite in the ink vehicle and the degree of dispersion of the smectite in the vehicle (which depends upon the particle size) in addition to the quality of the smectite itself. Accordingly, in order to obtain ink having desired flow characteristics, it is necessary to control not only the quality of the smectite but also the particle size and the degree of dispersion of the smectite.
However, though the quality of the smectite has been studied, the particle size of the smectite has been hardly studied. Further, since the smectite absorbs water to disintegrate and swell when introduced into water, there has not been established a means for controlling dispersion of the smectite.
Further, it has been conceived that the flow control effect of the smectite owes to the phenomenon that a gel structure is formed by hydrogen bonding of the ends of hydroxyl groups on a side of a leaf of the smectite and the leafs are cross-linked by molecules of water. Accordingly, it is conceived that if the conventional smectite addition technique for stencil printing emulsion ink is simply applied to stencil printing oil ink containing no water such as UV ink, the flowability of the ink cannot be stabilized.
In view of the foregoing observations and description, the primary object of the present invention is to provide a stencil printing ink containing therein a dispersant which can be applied to both stencil printing emulsion ink and stencil printing oil ink to stably disperse smectite in an improved manner.
The stencil printing ink of the present invention is characterized by containing therein a polyamino-amide dispersant and a smectite.
In this specification, the term xe2x80x9csmectitexe2x80x9d should be broadly interpreted not to mean a particular kind of smectite but to mean various natural or synthetic clay minerals such as bentonite, hectorite, heide llite, saponite and the like which contain montmorillonite as a main component and have been used as a flow control agent to be contained in known stencil printing inks.
The term xe2x80x9cpolyamino-amide dispersantxe2x80x9d means a dispersant containing therein a polyamino-amide compound. For example, the xe2x80x9cpolyamino-amide dispersantxe2x80x9d may be a polyamino-amide compound itself, or a polyamino-amide compound dissolved in solvent. In this specification, the term xe2x80x9ca polyamino-amide compoundxe2x80x9d should be broadly interpreted to include various compounds of polyamino-amide such as carboxylates of polyamino-amide, salts of polyamino-amide and esters, and the like.
The term xe2x80x9cstencil printing inkxe2x80x9d means ink for use in stencil printing and should be broadly interpreted to include both emulsion inks for the stencil printing and oil inks for the stencil printing. When the stencil printing ink is an emulsion ink, it is preferred that the polyamino-amide dispersant comprises a polycarboxylate of polyamino-amide, and when the stencil printing ink is an oil ink, it is preferred that the polyamino-amide dispersant comprises a salt of long-chain polyamino-amide and polar acid ester.
It is preferred that the polyamino-amide dispersant be contained in the ink in such an amount that makes the polyamino-amide compound content in the ink 0.05 to 5% by weight of the total weight of the ink, more preferably 0.1 to 3% by weight and most preferably 0.5 to 1.5% by weight.
The polyamino-amide content of the polyamino-amide dispersant is preferably 0.05 to 5% by weight of the total weight of the ink, more preferably 0.1 to 3% by weight and most preferably 0.5 to 1.5% by weight.
In the stencil printing ink in accordance with the present invention, by virtue of the polyamino-amide dispersant contained therein, particles of the smectite can be perfectly dispersed and stable flow control effect can be obtained. It is conceived that this is because the carboxyl group of the polyamino-amide and the smectite causes selective electrostatic chemisorption during wetting and a gel structure is formed, whereby the particle size of the smectite in the ink vehicle is made proper, and dispersion of the smectite particles in the ink vehicle is improved and stabilized.
When the particle size of the smectite in the ink vehicle is thus made proper, and dispersion of the smectite particles in the ink vehicle is thus improved and stabilized, the flow control effect by the smectite is enhanced, and a desired yield value of the ink (a minimum shearing force required to cause flow of the ink, or the degree to which the ink keeps its shape) can be obtained from the beginning without passing the ink composition through the roll mill increased times. (The times at which the ink composition is passed through the roll mill before a desired yield value is obtained will be referred to as the xe2x80x9croll pass timexe2x80x9d, hereinbelow.) That is, in the conventional ink without polyamino-amide dispersant, a sufficient flow control effect of the smectite cannot be obtained and accordingly, the yield value tends to be proportional to the roll pass time, that is, as the roll pass time is smaller, the yield value is lower and as the roll pass time is larger, the yield value is higher. To the contrast, in the stencil printing ink of the present invention, the dispersion of the smectite is improved and stabilized by virtue of the polyamino-amide dispersant, the yield value of the ink hardly depends upon the roll pass time and a desired yield value can be obtained at a smaller roll pass time.
Further, when the dispersion of the smectite is improved, the dispersion of other ink components such as coloring agents, especially phthalocyanine pigments which have been said to be very difficult to disperse, can be improved, whereby the quality of the ink can be improved.
Further, since sufficient dispersion can be obtained even if the roll pass time is one, the dispersion time can be shortened and the productivity of the stencil printing ink can be improved.
When the polyamino-amide compound content of the polyamino-amide dispersant is in the range of 0.05% to 5% by weight of the total weight of the ink, the particle size of the smectite in the ink vehicle is made further proper, and dispersion of the smectite particles in the ink vehicle is further improved and stabilized.